This area will cover relevant news of the threat to the planet from Near Earth Objects (NEOs) including concepts and designs for mitigation. All opinions are those of the author.

10 November 2008

JPL Update on 2008 TC3

Figure 1a. The terminal trajectory for Earth impacting asteroid 2008 TC3. The view is looking down on the ecliptic plane.

Figure 1b. The terminal trajectory for Earth impacting asteroid 2008 TC3. The view is from the sun. Note that the asteroid enters Earth shadow at about 1:49 UT so that the final portion of the trajectory is behind the Earth.

Figure 2. The predicted path of the asteroid is noted at 10 km intervals from 100 to 0 km altitude, neglecting atmospheric drag. The red dots indicate the reported instances of atmospheric entry at 65.4 km altitude and for the airburst at 37 km.

Figure 3,. Dispersion of 900 variant orbits, all of which could fit the existing observational data, at an altitude of 50 km. The nominal (most likely) position is denoted by the red dot in the center of the diagram. The results of this Monte Carlo simulation demonstrate that most possible orbits fell within 1 km from the mean, which is at (31.804°E, 20.858°N). The 1-, 2- and 3-sigma uncertainty ellipses are noted in red. Statistically and respectively, these ellipses capture 39%, 86% and 99% of the cases. The time of impact uncertainty at a given altitude is 0.16 seconds, 1-sigma.

Figure 4. Meteosat 8 / EUMETSAT infrared image of the 2008 TC3 explosion. The scale at the right gives the image intensity. Copyright 2008 EUMETSAT

From JPL, update on 2008 TC3:


A spectacular fireball lit up the predawn sky above Northern Sudan on October 7, 2008. This explosion was caused by the atmospheric entry of a small near-Earth asteroid, estimated to be no more than a few meters in diameter. The explosion likely scattered small meteorite fragments across the Nubian desert below. Although such small impact events occur several times per year around the globe, this case was unprecedented because the asteroid was actually discovered the day before it reached the Earth and the impact location and time were for the first time predicted in advance.

At 6:39 UT (UT = GMT) on the morning of October 6, 2008, Richard Kowalski, at the Catalina Sky Survey, discovered this small near-Earth asteroid using the Mt. Lemmon 1.5 meter aperture telescope near Tucson, Arizona. When the discovery observations were reported to the Minor Planet Center (MPC) in Cambridge Massachusetts, a preliminary orbit computation immediately indicated that the object was headed for an Earth impact within 21 hours. The MPC quickly made the discovery and subsequent "follow-up" observations available to the astronomical community and contacted the NASA/JPL Near-Earth Object Program Office. The MPC also notified NASA Headquarters of the impending impact so that subsequent US government interagency alerts and inter-governmental notifications could begin. By the time this object (now designated as 2008 TC3) entered the Earth's shadow 19 hours after discovery, some 570 astrometric (positional) measurements had been reported from 26 observatories around the world, both professional and amateur.

Within an hour of receiving the initial data set, JPL predicted that the object would enter the Earth's atmosphere above northern Sudan around 02:46 UT on October 7. As the day progressed and more and more data arrived from the MPC, JPL continued to improve the orbit for 2008 TC3 and forwarded updated predictions to NASA Headquarters. On the afternoon of Oct 6th, NASA Headquarters alerted officials at the National Security Council, the Office of Science and Technology Policy, the Department of State, and the Department of Defense Northern Command and Joint Space Operations Center. NASA also issued a press release at approximately 21:30 UT announcing the predicted impact later that night.

Detections of the actual atmospheric impact event suggested that it was an airburst explosion at an altitude of 37 km with an energy equivalent to about one kiloton of TNT explosives. The time and place of the predicted impact agree very well with a number of atmospheric entry observations including those from U.S. government satellites, infrasound signals from at least one ground station, images from the Meteosat 8 weather satellite and a sighting by a KLM airline pilot flying over Chad. The latest JPL trajectory estimate, which carefully considers all available data, including some measurements not available until after the event, is accurate to within a few kilometers at the time of atmospheric entry.

This dramatic prediction of an actual impact underscored the successful evolution of the Near-Earth Object (NEO) Program's discovery and orbit prediction process. The discovery was made, observations were provided by 26 international observatories, the orbit and impact computations were determined, verified and announced well before the impact, which took place only 20.5 hours after the discovery itself. While improvements to the impact prediction process still need to be made, the system worked well for the first predicted impact by a near-Earth object.

Asteroid 2008 TC3 Strikes Earth: Predictions and Observations Agree
Steve Chesley, Paul Chodas, and Don Yeomans
November 4, 2008

Link: JPL NEO News Release

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